Rheology modifier

ABSTRACT

The present invention relates to a composition comprising a hydrophobically modified alkylene oxide polyurethane characterized by having a M, of from 50,000 to about 150,000 Daltons and a polydispersity of 2.5 to about 5.0. The present invention further relates to a multi-step process for the preparation of the hydrophobically modified polymer.

BACKGROUND OF THE INVENTION

The present invention relates to hydrophobically modified urethanepolymers, which are used as rheology modifiers in waterborne coatingsformulations.

Rheology modifiers are used in waterborne coatings formulations tocontrol viscosity over a wide shear rate range. They may be associative(they associate with the dispersed phase) or non-associative (theythicken the water phase). Associative thickeners may be derived fromnatural products such as hydrophobically modified cellulose ethers, orprepared from synthetic polymers such as hydrophobically modifiedethylene oxide urethane (HEUR) polymers. A typical description of HEURpolymers and their preparation can be found in US 2009/0318595 A1, whichdescribes forming a combination of linear and branched HEUR polymers byreacting a polyglycol, a hydrophobic alcohol, a diisocyanate, and atriisocyanate together in a one-pot reaction.

U.S. Pat. No. 4,155,892 (Emmons et al.) describes the preparation oflinear as well as branched HEUR polymers in separate examples.

Rheology modifiers are typically classified as either low shear rateviscosity builders (Stormer viscosity builders, also known as KUviscosity builders) or high shear rate viscosity builders (ICIbuilders). It is desirable to increase ICI viscosity and the efficiencyof ICI building rheology modifiers without concomitant increase in KUviscosity because such increase limits the formulator's ability to add aKU building rheology modifier to the formulation.

The commercially available HEUR polymers ACRYSOL™ 5000 and ACRYSOL™ 6000rheology modifiers (a trademark of The Dow Chemical Company or itsaffiliates) exhibit ICI viscosities of 1.75 and 2.40 respectively and KUviscosities of 79.3 and 109.7 respectively in an acrylic semi-glosswhite paint. It would be advantageous to obtain a hydrophobicallymodified urethane polymer formulation with an increased ICI/KU viscosityratio in comparison to these known rheology modifiers.

SUMMARY OF THE INVENTION

In a first aspect, the present invention is a composition comprising ahydrophobically modified alkylene oxide polyurethane characterized byhaving a M_(w) of from 50,000 to about 150,000 Daltons and apolydispersity of 2.5 to about 5.0.

In a second aspect, the present invention is a process comprising thesteps of:

a) contacting a polyisocyanate branching agent with a stoichiometricexcess of a water-soluble polyalkylene glycol under such conditions toform a mixture of the polyalkylene glycol and a branchedpoly-hydroxyalkylene oxide urethane;b) contacting the mixture of the polyalkylene glycol and the branchedpoly-hydroxyalkylene oxide urethane with a difunctional linking agentselected from the group consisting of diisocyanates, epihalohydrins, andgem-dihalides, under such conditions to form a branched urethane polymercontaining hydroxyl-reactive end groups; andc) contacting the branched urethane polymer containing hydroxyl-reactiveend groups with a capping agent under such conditions to form ahydrophobically modified alkylene oxide urethane, wherein the cappingagent is characterized by either of the following formulas:

where R² is a C₄-C₃₀-alkyl, -aryl, or -aralkyl group; R³ is H or aC₄-C₃₀-alkyl, -aryl, or -aralkyl group; R⁴ is a C₄-C₃₀-alkyl, -aryl, or-aralkyl group; Y a C₂-C₈-alkylene group; W is N or P; x is an integerfrom 0 to 200; and Z is O or NH.

The polymer prepared by the process of the present invention has animproved ICI/KU viscosity balance over a polymer prepared using the samereagents and proportions by a standard 1-pot process.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a size exclusion chromatographic trace of acomparative HEUR composition.

FIG. 2 illustrates a size exclusion chromatographic trace of a HEURcomposition of the present invention.

FIG. 3 illustrates an overlay of the molecular weight distributions ofthe comparative HEUR composition and the HEUR composition of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect, the present invention is a composition comprising ahydrophobically modified alkylene oxide polyurethane characterized byhaving a M_(w) of from 50,000 to about 150,000 Daltons and apolydispersity of 2.5 to about 5.0.

The hydrophobically modified alkylene oxide polyurethane composition ofthe present invention is characterized, in part, by hydrophilicpolyalkylene oxide groups, examples of which include hydrophilicpolyethylene oxide, hydrophilic polyethylene oxide/polypropylene oxidecopolymers, and hydrophilic polyethylene oxide/polybutylene oxidecopolymers. A preferred hydrophobically modified alkylene oxidepolyurethane is a hydrophobically modified ethylene oxide polyurethane(HEUR).

The hydrophobically modified alkylene oxide polyurethane composition isfurther characterized by urethane groups that link the polyalkyleneoxide groups, preferably polyethylene oxide groups. The compositioncomprises a mixture of linear and branched polymers.

The hydrophobically modified alkylene oxide polyurethane composition isfurther characterized by inclusion of pendant hydrophobic cappinggroups, which are characterized by either of the following formulas:

where R² is a C₄-C₃₀-alkyl, -aryl, or -aralkyl group; R³ is H or aC₄-C₃₀-alkyl, -aryl, or -aralkyl group; R⁴ is a C₄-C₃₀-alkyl, -aryl, or-aralkyl group; Y a C₂-C₈ alkylene group; W is N or P; x is an integerfrom 0 to 200; and Z is O or NH.

In another embodiment, the composition of the present invention has aM_(w) of from 70,000 to 90,000 Daltons and a polydispersity of 2.9 toabout 3.3.

In another embodiment, the composition of the present invention has aM_(w) of from 110,000 to 140,000 Daltons and a polydispersity of 4.2 toabout 4.4.

In a second aspect, the present invention is a process comprising thesteps of:

a) contacting a polyisocyanate branching agent with a stoichiometricexcess of a water-soluble polyalkylene glycol under such conditions toform a mixture of the polyalkylene glycol and a branchedpoly-hydroxyalkylene oxide urethane;b) contacting the mixture of the polyalkylene glycol and the branchedpoly-hydroxyalkylene oxide urethane with a difunctional linking agentselected from the group consisting of diisocyanates, epihalohydrins, andgem-dihalides, under such conditions to form a branched urethane polymercontaining hydroxyl-reactive end groups; andc) contacting the branched urethane polymer containing hydroxyl-reactiveend groups with a capping agent under such conditions to form ahydrophobically modified alkylene oxide urethane, wherein the cappingagent is characterized by either of the following formulas:

where R² is a C₄-C₃₀-alkyl, -aryl, or -aralkyl group; R³ is H or aC₄-C₃₀-alkyl, -aryl, or -aralkyl group; R⁴ is a C₄-C₃₀-alkyl, -aryl, or-aralkyl group; Y a C₂-C₈ alkylene group; W is N or P; x is an integerfrom 0 to 200; and Z is O or NH.

In the first step of the process of the present invention, astoichiometric excess of a water-soluble polyalkylene glycol iscontacted with a polyisocyanate branching agent under conditions to forma poly-hydroxyethylene oxide urethane with unreacted polyethyleneglycol. A water-soluble polyalkylene glycol refers to water-solublepolyethylene oxides, water-soluble polyethylene oxide/polypropyleneoxide copolymers, water-soluble polyethylene oxide/polybutylene oxidecopolymers, and polyethylene oxide/polypropylene oxide/polybutyleneoxide terpolymers. As used herein, the term propylene oxide refers toeither a polymer having —(OCH₂CH-₂CH₂)— and/or —(OCH(CH₃)CH₂)— repeatinggroups.

Preferred water-soluble polyalkylene oxides are polyethylene glycols,particularly polyethylene glycols having a weight average molecularweight in the range of from 4000, more preferably from 6000, and mostpreferably from 7000, to 20,000, more preferably to 12,000 and mostpreferably to 9000 Daltons.

An example of a suitable polyethylene glycol is PEG 8000, which iscommercially available as CARBOWAX™ 8000 Polyethylene Glycol (atrademark of The Dow Chemical Company (“Dow”) or an affiliate of Dow,Midland, Mich.).

As used herein the term polyisocyanate branching agent is a compoundcontaining at least three isocyanate groups. Examples of a preferredclass of polyisocyanate branching agents include cyanurate trimers andbiuret trimers, which are characterized by the following formulas:

where R is a C₄-C₃₀-alkylene group, more particularly a C₄-C₂₀-alkylenegroup. As used herein, the term “alkylene group” refers to a biradicalsaturated or partially saturated hydrocarbyl group that is linear-,branched-, or cycloaliphatic or a combination thereof. Specific examplesof suitable cyanurate trimers, which are preferred, include HDIisocyanurate (trimer), and IPDI isocyanurate (trimer). The structures ofthese cyanurate compounds are illustrated:

Persons of ordinary skill in the art recognize that isocyanurate trimersgenerally include lower levels of oligomers (pentamers, heptamers, etc.)that impart higher order isocyanate functionality to the compound.Accordingly, the term “isocyanate trimer” may include the trimer byitself or as a mixture of trimer and other oligomers.

A stoichiometric excess of the polyalkylene glycol is advantageouslycontacted with the polyisocyanate branching agent in the presence of acatalyst designed to promote the coupling of the hydroxyl and isocyanategroups. Suitable catalysts include tin catalysts such as dibutyltindilaurate and dibutyltin diacetate. The reaction is carried out underconditions designed to ensure complete or substantially completeconsumption of the isocyanate groups. The polyisocyanate branchingreagent is conveniently added to a pre-dried mixture of the polyalkyleneglycol and optionally a non-interfering solvent with a relatively highboiling point such as toluene.

The mole equivalent:mole equivalent ratio of polyethyleneglycol:polyisocyanate branching agent is preferably at least 2:1, morepreferably at least 3:1, and most preferably at least 5:1, andpreferably not greater than 20:1, more preferably not greater than 10:1.

In a second step, the resultant mixture of the unreacted polyalkyleneglycol and the branched poly-hydroxyalkylene oxide urethane is contactedwith a difunctional linking agent under conditions sufficient to form abranched urethane polymer containing hydroxyl-reactive end groups,generally at a temperature in the range of 80 to 120° C. and over aperiod of 1 to 3 hours. As used herein, the term “difunctional linkingagent” refers to a C₄-C₂₀ diisocyanate, an epihalohydrin, or agem-dihalide, or combinations thereof. Diisocyanates, which arepreferred, may be aliphatic or aromatic diisocyanates, or combinationsthereof. As used herein, “aliphatic” refers to saturated or partiallyunsaturated linear-, branched-, or cycloaliphatic, or combinationsthereof. Examples of suitable diisocyanates include 1,4-tetramethylenediisocyanate, 1,6-hexamethylene diisocyanate,2,2,4-trimethyl-1,6-diisocyanatohexane, 1,10-decamethylene diisocyanate,4,4′-methylenebis(isocyanatocyclohexane), 1,4-cyclohexylenediisocyanate, isophorone diisocyanate, m- and p-phenylene diisocyanate,2,6- and 2,4-toluene diisocyanate, xylene diisocyanate,4-chloro-1,3-phenylene diisocyanate, 4,4′-biphenylene diisocyanate,4,4′-methylene diphenylisocyanate, 1,5-naphthylene diisocyanate, and1,5-tetrahydronaphthylene diisocyanate. Preferred diisocyanates includehexamethylene diisocyanate and isophorone diisocyanate.

Epihalohydrins include epichlorohydrin and epibromohydrin; gem-dihalidesinclude gem-dichlorides and gem-dibromides such as dichloromethane,dibromomethane, 1,1-dichloroethane, 1,1-dibromoethane,1,1-dichlorotoluene, and 1,1-dibromotoluene.

The difunctional linking agent is conveniently added to the potcontaining the branched poly-hydroxyalkylene oxide urethane/polyalkyleneglycol mixture at sufficient quantities and under conditions to ensurecomplete or substantially complete conversion of the hydroxyl groups tourethane groups. Thus, a stoichiometric excess of the difunctionallinking agent is conveniently used in this step.

The resultant branched urethane polymer containing hydroxyl-reactive endgroups is then contacted with a sufficient amount of a capping agentunder conditions to ensure substantially complete conversion ofisocyanate groups to urethane groups, generally at a temperature in therange of about 70 to 120° C. As used herein, the term “capping agent”refers to a compound of either of the following formulas:

where R²—R⁴, W, Y, Z, and x are as previously described. Capping agentsinclude linear, branched, or cycloaliphatic alcohols having at least 6carbon atoms, aromatic alcohols, or aralkyl alcohols, examples of whichinclude as n-hexanol, n-octanol, n-decanol, n-dodecanol, n-hexadecanol,2-ethyl-1-hexanol, 2-butyl-1-octanol, 2-butyl-1-decanol,2-hexyl-1-octanol, 2-hexyl-1-decanol, isononyl alcohol, isodecylalcohol, isoundecyl alcohol, nonyl phenol, cyclohexanol, and benzylalcohol; alkylamines, such as hexylamine, octylamine, and decylamine;and surfactants, such as ethoxylated alkanols, ethoxylated alkylphenols,and polyalkylene glycol monoalkyl ethers.

In another aspect, the capping agent is a C₈-C₂₀ linear or branchedalcohol; in another aspect, the capping agent is a C₈-C₂₀-alcohol-C₂-C₄—alkoxylate, preferably a C₈-C₂₀-alcohol-ethoxylate.

Volatile compounds are advantageously removed by convenient means suchas rotary evaporation and the desired polymer is isolated. It hassurprisingly been discovered that multi-step approach described hereingives a polymer with superior KU and ICI viscosities as compared topolymers prepared in a single pot reaction.

The coating composition according to the present invention may furtherinclude one or more of the following additives: Solvents; fillers;pigments, such as titanium dioxide, mica, calcium carbonate, silica,zinc oxide, milled glass, aluminum trihydrate, talc, antimony trioxide,fly ash, and clay; polymer encapsulated pigments, such aspolymer-encapsulated or partially encapsulated pigment particles such astitanium dioxide, zinc oxide, or lithopone particles; polymers orpolymer emulsions adsorbing or bonding to the surface of pigments suchas titanium dioxide; hollow pigments, including pigments having one ormore voids; dispersants, such as aminoalcohols and polycarboxylates;surfactants; defoamers; preservatives, such as biocides, mildewcides,fungicides, algaecides, and combinations thereof; flow agents; levelingagents; and additional neutralizing agents, such as hydroxides, amines,ammonia, and carbonates.

For example, the coatings composition may include polymer-encapsulatedopacifying pigment particles comprising i) opacifying pigment particles,such as titanium dioxide particles, having a diameter in the range of100 nm to 500 nm and an index of refraction of at least 1.8; ii) anencapsulating polymer, and iii) a polymeric dispersant for theencapsulated opacifying pigment particles and the polymer. Suchpolymer-encapsulated opacifying pigment particles are described, forexample, in U.S. Patent Publication US 2010/0298483 A1. In anotherexample, the coating composition may include polymer-encapsulatedopacifying pigment particles as described in WO 2007/112503A1.

DETAILED DESCRIPTION OF FIGURES

FIG. 1 illustrates an overlay of the SEC scan of the comparative sampleand a 10-point calibration curve generated from narrow molecular weightpolyethylene oxide standards.

FIG. 1 shows polymers eluting over retention times in the range of about15 min to about 25 min The weight average molecular weight (M_(w)) ofthis comparator was calculated to be 40,000 Daltons and itspolydispersity 1.8.

FIG. 2 illustrates an overlay of the SEC for the example of the presentinvention and a 10-point calibration curve generated from narrowmolecular weight polyethylene oxide standards. FIG. 2 shows polymerseluting over retention times in the range of about 14 mM to about 25min. The M_(w) of the example of the present invention was calculated tobe 78,000 Daltons and its polydispersity 3.1.

FIG. 3 illustrates an overlay of molecular weight distribution plots ofthe comparative sample (dotted plot) and Example 1 (continuous plot),obtained on three Asahipak columns (300×7.5 mm ID), pore size identifiedas: GF-310HQ, GF-510HQ, GF-710HQ, particle size 9 μm in a mobile phaseof 100 mM NH₄Ac in MeOH using R¹ detection.

Description of Thickener Evaluation in Paint

A latex paint composition, Pre-paint #1, was prepared by combining thefollowing components:

Kronos 4311 titanium dioxide slurry 262.8 g Water 180.1 g Ethyleneglycol 24.3 g Ropaque Ultra plastic pigment 49.7 g Rhoplex SG-30 binder420.9 g Drewplus L-475 defoamer 4.0 g Texanol coalescent 19.2 g TritonX-405 surfactant 2.5 g Total 963.5 g Kronos 4311 is a product of KronosIncorporated (Chelmsford, MA, USA). Drewplus L-475 defoamer is a productof Ashland Specialty Chemical Company. TRITON ™ X-405 stabilizer,ROPAQUE ™ Ultra opaque polymer and RHOPLEX ™ SG-30 emulsion are productsof The Dow Chemical Company or its subsidiaries.

The formulated paint was obtained by adding aqueous thickener dispersionand water to Pre-paint #1 (963.5 g). To maintain constant solids of thefully formulated paint, the combined weight of added thickener and waterwas 49.5 g. The density of the fully formulated paint was 1013pounds/100 gal (1.2 Kg/1). The pH of the fully formulated paints was inthe range of 8.5 to 9.0.

Formulated paints were made by the following method. To 963.5 gPre-paint #1 were added slowly aqueous thickener dispersion (40.0 g) andwater (9.5 g). The mixture was stirred for 10 min. The aqueous thickenerdispersions were made by adding dried solid thickener wax (10.0 g),methyl-beta-cyclodextrin (available from Wacker-Chemie Gmbh, Munich,Germany, 1.0 g of a 50% solution) and water (39.0 g) to a 50 mL plasticcentrifuge tube. The tubes were capped and mounted on a rotator forcontinuous tumbling over 48 h until the aqueous thickener dispersionswere homogeneous. Following a 24 h equilibration at room temperature,the thickened paint was stirred for 1 min on a lab mixer beforemeasuring viscosity values.

“KU viscosity” is a measure of the mid-shear viscosity as measured by aKrebs viscometer. The Krebs viscometer is a rotating paddle viscometerthat is compliant with ASTM-D562. KU viscosity was measured on aBrookfield Krebs Unit Viscometer KU-1+ available from BrookfieldEngineering Labs (Middleboro, Mass., USA).

“ICI viscosity” is the viscosity, expressed in units of poise, measuredon a high shear rate, cone and plate viscometer known as an ICIviscometer. An ICI viscometer is described in ASTM D4287. It measuresthe viscosity of a paint at approximately 10,000 sec⁻¹. ICI viscositiesof paints were measured on a viscometer manufactured by ResearchEquipment London, Ltd (London, UK). An equivalent ICI viscometer is theElcometer 2205 manufactured by Elcometer, Incorporated (Rochester Hills,Mich., USA). The ICI viscosity of a paint typically correlates with theamount of drag force experienced during brush application of the paint.

EXAMPLES

The following examples are for illustrative purposes only and notintended to limit the scope of the invention.

Comparative Example 1

A mixture of CARBOWAX™ 8000 Polyethylene Glycol (150 g) and toluene (360g) were added to a vessel and dried by azeotropic distillation. Themixture was cooled to 90° C. and Desmodur N3600 polyisocyanate (1.36 g)and Desmodur H (4.14 g) were added to the mixture. The mixture wasstirred for 5 min, then dibutyltin dilaurate (0.21 g) was added. Themixture was stirred for 1 h, then cooled to 80° C.; n-decanol (3.91 g)was then added and stirring was continued for 1 h. The mixture wascooled to 60° C. and the polymer was then isolated by rotaryevaporation. ICI and KU viscosities were found to be 1.90 Poise and 87.0Krebs units respectively. M_(w) as measured by SEC was found to be40,000 with M_(w)/M_(n) polydispersity equal to 1.9.

Comparative Example 2

A mixture of CARBOWAX™ 8000 Polyethylene Glycol (150 g) and toluene (360g) were added to a vessel and dried by azeotropic distillation. Themixture is cooled to 90° C. and Desmodur N3600 polyisocyanate (1.81 g)and Desmodur H (3.94 g) was added to the mixture. The mixture wasstirred for 5 minutes, then dibutyltin dilaurate (0.21 g) was added. Themixture was stirred for 1 hour whereupon n-decanol (3.91 g) was thenadded to the mixture; stiffing was continued for an additional 1 hour.The mixture was cooled to 60° C. and the polymer isolated via rotaryevaporation. ICI and KU viscosities were found to be 2.10 Poise and102.1 Krebs Units, respectively. M_(w) as measured by SEC was found tobe 43,000 with M_(w)/M_(n) polydispersity equal to 1.9.

Example 1

A of CARBOWAX™ 8000 Polyethylene Glycol (150 g) and toluene (360 g) wereadded to a vessel and dried by azeotropic distillation. The mixture wascooled to 90° C. and Desmodur N3600 polyisocyanate (1.36 g) was added tothe mixture. The mixture was stirred for 5 min, then dibutyltindilaurate (0.21 g) was added. The mixture was stirred for 1 h, afterwhich time Desmodur H (4.15 g) was added. Stiffing was continued for anadditional 1 h then cooled to 80° C. n-Decanol (3.91 g) was then addedto the mixture and stirring was continued for an additional 1 h. Themixture was cooled to 60° C. and the polymer was isolated by way ofrotary evaporation. ICI and KU viscosities were found to be 2.40 Poiseand 90.2 Krebs units respectively. M_(w) as measured by SEC was found tobe 79,000 with M_(w)/M_(n) polydispersity equal to 3.1.

Although the nature and amounts of the starting materials used to makethe polymers of Comparative Example 1 and Example 1 are the same, theICI/KU viscosity profile is clearly superior for Example 1, the productprepared by the process of the present invention, as evidenced by thehigher ICI/KU ratio for the product of the present invention.

Example 2

A mixture of CARBOWAX™ 8000 Polyethylene Glycol (150 g) and toluene (360g) were added to a vessel and dried by azeotropic distillation. Themixture is cooled to 90° C. and Desmodur N3600 polyisocyanate (1.81 g)was added to the mixture. The mixture was stirred for 5 minutes, thendibutyltin dilaurate (0.21 g) was added. The mixture was stirred for 1hour, after which time Desmodur H (3.94 g) was added. Stirring wascontinued for an additional 1 hour then cooled to 80° C. n-Decanol (3.91g) was then added to the mixture and stirring was continued for anadditional 1 hour. The mixture was cooled to 60° C. and the polymerisolated via rotary evaporation. ICI and KU viscosities were found to be2.80 Poise and 109.1 Krebs Units, respectively. M_(w) as measured by SECwas found to be 124,000 with M_(w)/M_(n) polydispersity equal to 4.4.

Again, the ICI/KU viscosity profile is superior for Example 2 ascompared to Comparative Example 2.

Size exclusion chromatography (SEC) was carried out for theabove-prepared samples as follows:

a) Sample Preparation:

Samples were prepared in 100 mM NH₄Ac in MeOH (Optima grade from Fisher)at concentration of about 2 mg/g. Samples were brought into solution byshaking on the mechanical shaker overnight at ambient temperature.Sample solutions were filtered using 0.45 μm PTFE filters. Samplesappeared to be soluble (checked visually) and no resistance was observedduring filtration process.

b) SEC Set-Up and Separation Conditions:

Separations were carried out on a liquid chromatograph consisting of anAgilent 1100 Model isocratic pump and injector (Waldbronn, Germany) andWaters 2414 Model differential refractometer (Milford, Mass.) operatedat 40° C. System control, data acquisition and data processing wereperformed using Cirrus° software version 3.1 (Polymer Laboratories,Church Stretton, UK).

The calibration curve was generated using polyethylene oxide standardswith the following M_(p)s: 615, 1500, 3930, 12140, 23520, 62100, 116300,442800, 909500 and 1258000. the polydispersities of these standardsranged from about 1.04 to 1.16. The standards were obtained commerciallyfrom Polymer Laboratories, part of Agilent, part# PL2080-0201. SECseparations were performed with a mobile phase of 100 mM NH₄Ac in MeOH(Optima grade from Fisher) @ 1 mL/min The SEC column set used in thisstudy was composed of three Asahipak columns (300×7.5 mm ID) packed withhighly cross-linked polar gel. The column pore size was identified as:GF-310HQ, GF-510HQ and GF-710HQ, the particle size was 9 μm and thecolumns were purchased from Shoko America (Torrance, Calif.).

In characterizing the composition of the present invention it isunderstood that the ranges used to calculate M_(w) and polydispersity ofthe composition are relative to the calibration standards, the columns,and the detector used to analyze and separate components of thecomposition as described above. Accordingly, as used herein, M_(w) andpolydispersity values and ranges refer to the M_(w) and polydispersitiesgenerated for samples under the SEC conditions described above.

1. A process comprising the steps of: a) contacting a polyisocyanatebranching agent with a stoichiometric excess of a water-solublepolyalkylene glycol under such conditions to form a mixture of thepolyalkylene glycol and a branched poly-hydroxyalkylene oxide urethane;b) contacting the mixture of the polyalkylene glycol and the branchedpoly-hydroxyalkylene oxide urethane with a difunctional linking agentselected from the group consisting of diisocyanates, epihalohydrins, andgem-dihalides, under such conditions to form a branched urethane polymercontaining hydroxyl-reactive end groups; and c) contacting the branchedurethane polymer containing hydroxyl-reactive end groups with a cappingagent under such conditions to form a hydrophobically modified alkyleneoxide urethane, wherein the capping agent is characterized by either ofthe following formulas:

where R² is a C₄-C₃₀-alkyl, -aryl, or -aralkyl group; R³ is H or aC₄-C₃₀-alkyl, -aryl, or -aralkyl group; R⁴ is a C₄-C₃₀-alkyl, -aryl, or-aralkyl group; Y a C₂-C₈-alkylene group; W is N or P; x is an integerfrom 0 to 200; and Z is O or NH.
 2. The process of claim 1 wherein thepolyalkylene glycol is a polyethylene glycol having a weight averagemolecular weight in the range of 4000 to 20,000 Daltons.
 3. The processof claim 1 wherein the polyisocyanate branching agent includes acyanurate trimer or a biuret trimer, or a combination thereof, whichtrimers are characterized by the following formulas:

where R is a C₄-C₃₀-alkylene group.
 4. The process of claim 1 whereinthe polyisocyanate branching agent includes HDI isocyanurate (trimer) orIPDI isocyanurate (trimer) or a combination thereof and the polyethyleneglycol has a weight average molecular weight in the range of 7000 to9000 Daltons.
 5. The process of claim 4 wherein the difunctional linkingagent is a C₄-C₂₀-diisocyanate.
 6. The process of claim 5 wherein thecapping agent is a C₈-C₂₀-alcohol.
 7. The process of claim 5 wherein thecapping agent is a C₈-C₂₀-alcohol-C₂-C₄— alkoxylate.
 8. A processcomprising the steps of: a) contacting HDI isocyanurate trimer with astoichiometric excess of polyethylene glycol having a weight averagemolecular weight in the range of 7000 to 9000 Daltons under suchconditions to form a mixture of the polyethylene glycol and a branchedpoly-hydroxyethylene oxide urethane; b) contacting mixture of thepolyethylene glycol and the branched poly-hydroxyethylene oxide urethanewith hexamethylene diisocyanate under such conditions to form a branchedurethane polymer containing isocyanate end groups; and c) contacting thebranched urethane polymer containing isocyanate end groups with a C₈-C₂₀alcohol under such conditions to form a hydrophobically modifiedethylene oxide urethane.
 9. The process of claim 8 wherein the cappingagent is n-decanol or 2-butyl-1-octanol.
 10. A composition comprising ahydrophobically modified alkylene oxide polyurethane characterized byhaving a M_(w) of from 50,000 to about 150,000 Daltons and apolydispersity of 2.5 to about 5.0.
 11. The composition of claim 10wherein the hydrophobically modified alkylene oxide polyurethane is ahydrophobically modified ethylene oxide polyurethane having a M_(w) offrom 70,000 to about 90,000 Daltons and a polydispersity of 2.9 to about3.3.
 12. The composition of claim 10 wherein the hydrophobicallymodified alkylene oxide polyurethane is a hydrophobically modifiedethylene oxide polyurethane having a M_(w) of from 110,000 to about140,000 Daltons and a polydispersity of 4.2 to about 4.6.
 13. Thecomposition of claim 10 which further includes polymer-encapsulated orpartially encapsulated opacifying pigment particles.